1. Field of the Invention
The present invention relates to a meter zero term of a Coriolis flowmeter, and more particularly, to an improved meter zero term.
2. Statement of the Problem
Vibrating conduit sensors, such as Coriolis mass flowmeters and vibrating densitometers, typically operate by detecting motion of a vibrating conduit that contains a flowing material. Properties associated with the material in the conduit, such as mass flow, density and the like, can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of the vibrating material-filled system generally are affected by the combined mass, stiffness, and damping characteristics of the containing conduit and the material contained therein.
A typical Coriolis mass flowmeter includes one or more conduits that are connected inline in a pipeline or other transport system and convey material, e.g., fluids, slurries, emulsions, and the like, in the system. Each conduit may be viewed as having a set of natural vibration modes, including for example, simple bending, torsional, radial, and coupled modes.
In a typical Coriolis mass flow measurement application, a conduit is excited in one or more vibration modes as a material flows through the conduit, and motion of the conduit is measured at points spaced along the conduit. Excitation is typically provided by an actuator, e.g., an electromechanical device, such as a voice coil-type driver, that perturbs the conduit in a periodic fashion. Two transducers (or pickoff sensors) are typically employed in order to measure a vibrational response of the flow conduit or conduits, and are typically located at positions upstream and downstream of the actuator. Mass flow rate may be determined by measuring time delay or phase differences between motions at the spaced-apart transducer locations, wherein the time delay or phase difference are caused by Coriolis forces in the flowing material. The Coriolis forces are generated by the directional change in the moving fluid due to the tube vibrations. These Coriolis forces are exerted on the sensor tube and produce perturbations in the vibrational motion. These perturbations will cause one end of a flowtube to lead and the other end to lag, creating a phase delay in the leading and lagging vibration sensor signals.
The pickoff sensors are connected to meter electronics (or other instrumentation) that receives the signals from the pickoff sensors and processes the signals in order to derive a mass flow rate measurement, among other things. To generate a mass flow rate measurement, the meter electronics can convert the measured phase delay into a time delay using the driving frequency of the vibration. The mass flow rate passing through the flow tubes is directly proportional to this time delay (Δt), as given by:mass flow rate=FCF×Δt  (1)
The (FCF) term is a flow calibration factor that takes into account various meter characteristics such as meter stiffness, ambient temperature, and meter construction and geometry, for example. However, in actual operation at a no flow condition, the time delay (Δt) may comprise a non-zero value and must be compensated for in the equation to accurately measure flows. Consequently, the mass flow rate may be better represented as:mass flow rate=FCF×(Δt−Δtz)  (2)
The (Δtz) term is a time delay correction value at a no-flow condition, also called a meter zero term. The meter zero term (Δtz) may generate a no-flow vibrational phase shift due to positional, mass, and/or damping asymmetries between the driver and the pickoff sensor or sensors. The meter zero term (Δtz) may also exist due to modal interactions of a pickoff sensor with the driving mode of the flowtube or tubes. The meter zero term (Δtz) may exist due to pickoff sensor and driver design. The meter zero term (Δtz) may exist due to environmental temperature and changes in the temperature.
It is well known in the art that the meter zero term (Δtz) and the stability of the meter zero term (Δtz) is greatly affected by geometric asymmetries of the flowtubes and/or the flowmeter assembly as a whole, by coupling between vibrational modes, by damping, and by the meter mounting characteristics and other environmental conditions.
These factors not only contribute to the magnitude of the meter zero term (Δtz), but may also cause instability in the meter zero term (Δtz) over time. This in turn affects the accuracy of the flow meter, especially at higher turn down. Meter turn down comprises a band of low flow rates just above a zero flow where the measurement signal cannot be distinguished from noise, i.e., flows too low to be accurately measured.
For these reasons, it is desired to keep the meter zero term (Δtz) as small as possible. A large meter zero term (Δtz) may present problems in a vibratory flowmeter. A meter zero term (Δtz) of large magnitude may be more unstable than a meter zero term (Δtz) of small magnitude. A meter zero term (Δtz) of large magnitude may require more frequent re-zeroing operations.
A re-zeroing operation will require taking the vibratory flowmeter out of operation. The re-zeroing operation may require manual and time-consuming diagnostics/adjustments by a technician. For example, the user of the flowmeter is typically required to re-zero the flowmeter when the temperature changes by more than 20 degrees Centigrade.
Although the temperature effect on the meter zero term (Δtz) is compensated for in the factory calibration process, the meter zero term (Δtz) is typically non-adjustable. The stability of the meter zero term (Δtz) is not capable of being adjusted or compensated.